1. Field of the Invention
The present invention relates to improved processes and apparatus for removal and recovery of copper, particularly from solutions such as spent etchant-type solutions produced in printed circuit (wiring) board manufacture.
2. Background
Multilayer printed circuit boards are used for a variety of electrical applications and provide the advantage of conservation of weight and space. A multilayer board is comprised of two or more circuit layers, each circuit layer separated from another by one or more layers of dielectric material. Circuit layers are formed by applying a copper layer onto a polymeric substrate. Printed circuits are then formed on the copper layers by techniques well known to the art, for example print and etch to define and produce the circuit tracesxe2x80x94i.e., discrete circuit lines in a desired circuit pattern. Once the circuit patterns are formed, a stack is formed comprising multiple circuit layers separated from each other by a dielectric layer, typically epoxy. Thereafter, the stack is subjected to heat and pressure to form the laminated multilayer circuit board.
Following lamination, the multiple circuit layers are electrically connected to each other by drilling through holes through the board surface. Resin smear from through-hole drilling is removed under rather stringent conditions, for example, by treatment with a concentrated sulfuric acid or hot alkaline permanganate solution. Thereafter, the through-holes are further processed and plated to provide a conductive interconnecting surface.
Prior to lamination and through hole formation, the discrete copper circuit lines are typically treated with an adhesion promoter to improve bond strength between each circuit layer and adjacent interleaving dielectric resin layers. One method used by the art to improve bond strength involves oxidative treatment of the copper circuit lines to form a copper oxide surface coating on the circuit lines. The oxide coating is usually a black or brown oxide layer that adheres well to the copper. The oxide possesses significantly more texture or roughness than an untreated copper surface. Chemical treatments which produce adherent conversion coatings on metal surfaces, such as black oxide, are very commonly used as to promote adhesion of organic materials to metals. Other examples include metal phosphate coatings used as paint adhesion promoters. Such roughened and conversion coated surfaces enhance adhesion and wettability to the adjacent insulating layer by a mechanism that is believed to include mechanical interlocking between the metal surface and a dielectric resin layer. Metal surfaces that have been microetched, but not conversion coated do not generally possess as high a degree of surface roughness and texture, as can be inferred from their greater reflection of visible light.
Other techniques known in the art to promote adhesion between copper surfaces and dielectric resins prior to multilayer lamination include the use of etches inclusive of cupric chloride etchants, mechanical treatments designed to produce surface texture, and metal plating, all designed to produce roughened surfaces.
Highly useful compositions for treatment of copper and its alloys to form an etched surface suitable for lamination of circuit layers in multilayer circuit fabrication have been disclosed in U.S. patent application Ser. No. 09/198880, filed Nov. 24, 1998, and corresponding South Korean patent application no. 9854330, and Japanese Application 37764/1998, Publication Number 10-377764/1998, all assigned to the Shipley Company. The disclosed compositions are acidic aqueous solutions and contain inter alia a triazole, preferably benzotriazole together with a peroxide, preferably hydrogen peroxide.
While such etchant solutions are highly effective, the compositions also yield a waste solution that contains copper. Such spent etchant solutions may be undesirable from a waste treatment/cost perspective. For example, copper waste solutions have been disposed of by off-site contractor or generate large quantities of sludge. In either approach, the spent solutions and/or their contained metals are typically eventually consigned to landfill.
Clearly, practical alternative methods of treatment or disposal of such solutions containing copper would be very desirable.
The present invention provides novel processes and apparatus for removal of copper from a solution or fluid, particularly spent etchant solutions that contain copper produced through printed circuit board manufacture.
The invention is particularly useful for recovery/removal of copper from aqueous admixtures that contain a copper complexing agent, e.g. a nitrogen-containing compound including cyclic compounds such as an azole, particularly a tetrazole, triazole or thiadiazole, or other agent that can complex with copper such as non-cyclic compounds, particularly amines, preferably secondary or tertiary amines which may optionally have acidic moieties such as EDTA. The complexing agent also may be aromatic such as benzotriazole or benzothiadiazole. The invention is particularly useful for recovery/removal of copper from aqueous admixtures that contain a benzotriazole, or other triazole. Systems of the invention can be characterized at least in part by use of a separated (e.g. interposed ion-permeable membrane) electrolytic copper removal from such aqueous admixtures.
It has been surprisingly found that copper can not be effectively removed from solutions containing a copper complexing agent such as benzotriazole by many methods such as precipitation and simple plating. However, it has been found that copper can be removed most effectively from such solutions using a separated cell system, enabling convenient treatment of copper etchant solutions.
Without being bound by any theory, it is currently thought that other methods of copper removal are unsuccessful as a result of the combination of the copper being present in complexed form with a complexing agent such as benzotriazole and the polarisation being conferred on any immersed cathode (on a simple plate-out procedure) by the complexing agent.
In particular detail, microetch solutions based on sulfuric acid and peroxide are formulated for application within the printed circuit board inner layer process manufacturing stages with relatively large amounts of contained complexant chemistry, e.g. a benzotriazole or other triazole. After use on a process line, such microetches generally become exhausted when the contained copper levels approach about 20 to 25 grams per litre. At that stage, the solution would be discarded and require treatment via disposal to satisfy local environmental compliance as is in effect in the United Kingdom and elsewhere.
By the processes and apparatus of the present invention, copper can be removed from such spent microetch solutions, and the treated solutions either re-used in a copper microetching solution, or the solution disposed of without the concern of copper content.
A wide variety of other solutions and other compositions containing copper may be treated in accordance with the invention. For instance, an aqueous admixture that contains copper, but not a copper complexing agent, may have a removal/purification effective amount of a complexing agent such as benzotriazole or EDTA added to the copper admixture and that admixture copper treated via a separated cell system in accordance with the invention. Relatively small amounts of a complexing agent can be added to a copper containing solution in order to effectively remove the copper in accordance with the invention, e.g. less than 20 or 10 mole percent of the complexing agent relative to the copper in the admixture. Larger amounts of the complexing agent also may be employed. Optimal amounts of complexing agent to be added to a copper containing admixture (e.g. aqueous copper solution) for removal of the copper can be determined by simple testing, e.g. various test samples of the copper admixture have differing amounts of the complexing agent added thereto can be treated via a separated cell system in accordance with the invention, and the treated sample then can be tested for copper content.
Copper removal systems of the invention include an electrodialysis unit, and the solution for copper recovery (e.g. a spent microetch solution) may be used as the anolyte or catholyte. Thus, for instance, if the microetch solution is used as the anolyte, a wide variety of compatible electrolytes may be used as the catholyte, e.g. acidic solutions of sulfuric acid, fluoroboric acid, methane sulphonic acid and the like. Preferably the solution to be treated (e.g. the microetch solution) is used as the anolyte, and an acidic solution that does not contain copper or a benzotriazole or other triazole or other copper complexing agent (e.g. a dilute H2SO4 solution) is used as the catholyte. For example, waste plating solutions that contain copper can be treated via a separated cell system in accordance with the invention to thereby remove copper therefrom.
It should be appreciated that although the present invention is generally discussed in the context of copper removal/recovery from aqueous solutions produced through printed circuit board manufacture, the invention also will be applicable to removal/recovery of copper from solutions and mixtures produced in other manufacturing processes.
It should be further appreciated that references herein to removal/recovery of copper from solutions are inclusive of removing those metals as may be present as dispersions or other forms in a solvent. Typically, solutions treated for metal removal/recovery in accordance with the invention will be aqueous solutions, but those solutions also may comprise an organic solvent component, particularly where the organic solvent is miscible with water.
The term xe2x80x9ccomplexing agentxe2x80x9d or xe2x80x9ccopper complexing agentxe2x80x9d as used herein refers to any compound that when present with can effect removal of copper from an aqueous admixture via a separated electroyltic cell as disclosed herein. Thus, complexing agents can be identified by simple testing, i.e. a candidate complexing agent can be added to an aqueous solution that contains copper, that solution passed through a separated electrolytic cell in accordance with the invention, and the treated solution analyzed for copper removal. The complexing agent will typically complex or otherwise interact with copper in the admixture.
Other aspects of the invention are disclosed infra.